We have previously demonstrated that select viral antigens, including human immunodeficiency virus (HIV) itself, can stimulate human macrophages (MO) to secrete monokines capable of upregulating HIV expression in chronically infected human cells. Subsequent investigations to determine which HIV proteins were capable of stimulating monokine production revealed that the natural gp120 envelope protein could mediate this response, whereas the natural capsid protein could not. We found that natural gp120 stimulates MO to release TNF-alpha, IL-1beta, IL-6 and GM- CSF, and this effect could be blocked with soluble CD4. Analysis utilizing recombinant HIV proteins demonstrate that envelope proteins expressed in baculovirus, including gp160 and gp120, are unable to stimulate monokine secretion. Similarly, a recombinant gp120 fusion protein expressed in Chinese hamster ovary (CHO) cells is unable to function in this capacity. However, a full-length recombinant gp120 expressed in CHO cells is capable of stimulating monokine production. The stimulatory capacity of both natural and full-length CHO-cell derived gp120 was eliminated by heating at 100 degrees C, and could be blocked with excess CHO-cell derived recombinant gp120 fusion protein. Since the baculovirus-expressed gp120 and the CHO-cell derived recombinant fusion protein bind to CD4, these test results suggest that HIV gp120 binding to CD4 on the MO surface may of itself be insufficient for stimulation of monokine secretion.